Graphene is the two-dimensional crystalline form of carbon made of a single layer of carbon atoms arranged in hexagons, like a honeycomb. Graphene has attracted considerable interest as a new electronic material for fundamental studies and potential applications in future electronics due to its exceptional electronic characteristics. The room temperature carrier mobility of graphene devices can reach 15,000 cm2/V·s or higher, making it an attractive candidate for next generation electronics. However, two-dimensional graphene is a semi-metal with zero band gap and remains highly conductive even at the charge neutrality point, and therefore, cannot be used for field-effect transistors (FETs) at room temperature.
The formation of nanostructures with lateral quantum confinement can open up a finite band gap in graphene. In particular, graphene nanoribbons (GNRs) have been predicted to be semiconducting due to edge effects and the quantum confinement of electron wave functions in the transverse direction. Theoretical calculations have suggested that the band gap of GNRs scales inversely with their width, and a width in the sub 10 nanometer regime is required to create a sufficiently large band gap for room temperature transistor operation.
For example, a band gap of 0.67 eV (like Ge) requires a ribbon width of 2-3 nm. However, it is non-trivial to obtain GNRs in the sub 10 nm regime experimentally. Conventional electron beam (e-beam) lithography can produce GNRs of variable widths, and it has been demonstrated that the band gap of the GNRs indeed inversely scales with the width. However, the smallest GNR width that can be obtained using e-beam lithography is about 15-20 nm, which prevents this method from being used to obtain GNRs with a sufficiently large band gap for room temperature FETs.
Additionally, GNRs fabricated by e-beam lithography usually have a line edge roughness of 1-3 nm, which can adversely impact their electronic properties, and makes it practically impossible to obtain uniform GNRs in the sub 5 nm regime using conventional lithography. Interestingly, it has recently been shown that chemical exfoliation and sonication can produce GNRs with ultra-narrow widths down to 2-3 nanometers. See Jiao, L. Y., Zhang, L., Wang, X. R., Diankov, G. & Dai, H. J. Narrow Graphene Nanoribbons From Carbon Nanotubes, Nature 458, 877-880 (2009). Room temperature FETs have been demonstrated from these ultra-narrow GNRs. However, the GNRs obtained with this method came from a chance observation rather than through a specific fabrication process, and there is no clear pathway to rationally control the width of the resulting GNRs. Longitudinal unzipping of carbon nanotubes was also recently explored to produce GNRs around the 10 nm regime. In this regard, methods have been proposed to fabricate GNRs with controllable widths from 6-10 nm using chemical synthesized nanowires as etch masks. See Duan, X. Assembled semiconductor nanowire thin films for high-performance flexible macroelectronics. MRS Bulletin 32, 134-141 (2007). Using such GNRs as the semiconducting channel, room temperature FETs have been fabricated with on-off ratios of >100.
However, the devices made from GNRs often have limited driving current, transconductance or frequency response determined by the intrinsic properties of individual GNRs, which can severely limit the capability to rationally design and fabricate devices to meet specific circuit requirements. To ensure sufficiently large driving current or transconductance for high frequency circuits will require the high-density assembly of multiple uniform GNRs in parallel, which has not been achieved to date. Additionally, integration of nanoribbon devices into a useful circuit will also require large scale hierarchical assembly of nanoribbons into highly organized arrays over multiple length scales. These assembly requirements are highly elusive considering the limited success to date in related topics on the assembly of nanotubes and nanowires after more than ten years of intensive research.